Many radiotelephones employ retractable antennas, i.e., antennas which are extendable and retractable out of the radiotelephone housing. The retractable antennas are electrically connected to a signal processing circuit positioned on an internally disposed printed circuit board. In order to optimally operate, the signal processing circuit and the antenna should be interconnected such that the respective impedances are substantially "matched", i.e., electrically tuned to filter out or compensate for undesired antenna impedance components to provide a 50 Ohm impedance value at the circuit feed. Unfortunately, complicating such a matching system, a retractable antenna by its very nature has dynamic components, i.e., components which move or translate with respect to the housing and the printed circuit board and as such does not generally have a single impedance value. Instead, the retractable antenna typically generates largely different impedance values when in an extended versus a retracted position. Therefore, it is preferred that the impedance matching system alter the antenna's impedance to properly match the terminal's impedance both when the antenna is retracted and extended.
The physical configuration of the matching network is further complicated by the miniaturization of the radiotelephone and the internally disposed printed circuit board. Many of the more popular hand-held telephones are undergoing miniaturization. Indeed, many of the contemporary models are only 11-12 centimeters in length. Because the printed circuit board is disposed inside the radiotelephone, its size is also shrinking, corresponding to the miniaturization of the portable radiotelephone. Unfortunately, as the printed circuit board decreases in size, the amount of space which is available to support desired operational and performance parameters of the radiotelephone is generally correspondingly reduced. Therefore, it is desirable to efficiently and effectively utilize the limited space in the radiotelephone and on the printed circuit board.
This miniaturization can also create complex mechanical and electrical connections with other components such as the outwardly extending retractable antenna which must generally interconnect with the housing for mechanical support, and, as discussed above, to an impedance matching system operably associated with the printed circuit board in order for the signal to be processed.
Referring to FIGS. 1A and 1B, desired equivalent circuits 10, 10' are illustrated for extended and retracted antenna positions, respectively. As shown, in FIG. 1A, in the extended position the antenna rod 12 operates with a half-wave (.lambda./2) load. In this situation, the associated impedance may rise as high as 600 Ohms. In contrast, in the retracted position, as shown in FIG. 1B, the antenna rod 12 operates with a quarter-wave (.lambda./4) load with an impedance typically near 50 Ohms. Therefore, when the antenna is in the extended position an L-C matching circuit 15 may be needed.
In the past, conventional portable radiotelephones have used a variety of antenna connections to match the impedance in the antenna to the housing and the printed circuit board. For example, U.S. Pat. No. 5,374,937 to Tsunekawa et al. proposes downwardly spaced-apart contacts or terminals on the printed circuit board in the radiotelephone housing which act to engage with or short out of the associated matching network. Unfortunately and disadvantageously, this type of switching connection can employ a number of discrete switching components such as wiping contacts and additionally may use an undesirable amount of space on the printed circuit board. Further, this configuration can limit the operational bandwidth of the radiotelephone.
One alternative is described in a co-pending patent application, entitled "Radiotelephones with Antenna Matching Switching System Configurations" by Gerard J. Hayes and Howard E. Holshouser, identified by Attorney Docket No. 8194-73. This system employs transversely spaced-apart circuit and antenna contacts to reduce the amount of space on the printed circuit board needed to operate the matching system. However, the system employs a number of discrete components in the switching assembly and interconnection of the antenna to the circuit board of the device.